1. Field of the Invention
The present invention pertains to a refrigeration system and more specifically to the expansion valve of the refrigeration system that controls the expansion of the refrigerant between the condenser and the evaporator coils of the system.
2. Description of the Related Art
In a conventional refrigeration system, a liquid refrigerant is circulated through the system and absorbs and removes heat from an internal environment that is cooled by the system and then rejects that absorbed heat in a separate external environment.
FIG. 1 is a temperature (T) versus entropy (S) diagram of a conventional refrigeration cycle. In the conventional refrigeration cycle, refrigerant vapor enters the compressor at point 1 and is compressed to an elevated pressure at point 2. The refrigerant then travels through the condenser coil nearly at constant pressure from point 2 to point 3. At point 3, the elevated pressure of the refrigerant has a saturation temperature that is well above the ambient temperature of the external environment. As the refrigerant passes through the condenser coil the refrigerant vapor is condensed into a liquid. From point 3 to point 4 the liquid refrigerant is cooled further by about 10 degrees F. below the saturation temperature. After the condenser, from point 4 to point 5, the liquid refrigerant passes through an expansion valve and the liquid refrigerant is lowered in pressure to a liquid-vapor state, with the majority of the refrigerant being liquid. The expansion valve in the conventional refrigeration cycle is essentially an orifice. The decrease in pressure of the refrigerant is a constant enthalpy process. Entropy increases due to the mixing friction that occurs in the standard expansion valve. The cold refrigerant then passes through the evaporator coils from point 5 to point 1. A fan circulates the warm air of the internal environment across the evaporator coils and the coils gather the heat from the circulated air of the internal environment. The refrigerant vapor then returns to the compressor at point 1 to complete the refrigeration cycle.
FIG. 2 is a schematic representation of a standard refrigeration system. The standard system shown in FIG. 2 has four basic components: a compressor 6, a condenser 7, an expansion valve (also called a throttle valve) 8, and an evaporator 9. The system also typically includes an external fan 10 and an internal fan 11.
In the operation of the refrigeration system, the circulating refrigerant enters the compressor 6 as a vapor and is compressed to a high pressure, resulting in a higher temperature of the refrigerant. The hot, compressed vapor is then in the thermodynamic state known as a super-heated vapor. At this temperature and pressure, the refrigerant can be condensed with typically available ambient cooling air from the external environment of the refrigeration system.
The hot vapor is passed through the condenser where it is cooled in the condenser coils and condenses into a liquid. The external fan 10 moves the ambient air of the external environment across the condenser coils. The heat of the refrigerant passing through the condenser coils passes from the coils to the air circulated through the coils by the fan 10. As the heat of the refrigerant passes from the condenser coils into the circulating air, the refrigerant condenses to a liquid.
The liquid refrigerant then passes through the expansion valve 8 where the liquid undergoes an abrupt reduction in pressure which causes part of the liquid refrigerant to evaporate to a vapor. The evaporation lowers the temperature of the liquid and vapor refrigerant to a temperature that is colder than the temperature of the internal environment of the refrigeration system that is being cooled.
The cold liquid and vapor refrigerant are then routed through the evaporator coils. The internal fan 11 circulates the warm air of the internal environment across the coils of the evaporator 9. The warm air of the internal environment circulated by the fan 11 through the coils of the evaporator 9 evaporates the liquid part of the cold refrigerant mixture passing through the coils of the evaporator 9. Simultaneously, the circulating air passed through the coils of the evaporator 9 is cooled and lowers the temperature of the internal environment.
The refrigerant vapor exiting the coils of the evaporator 9 is routed back to the compressor 6 to complete the refrigeration cycle.
Air conditioning designers have for years increased the efficiency of the standard refrigeration cycle described above by several means. Some examples of those that have been successful include:                Use of “scroll” compressors that are more efficient than screw or piston-type compressors.        Use of high efficiency compressor motors such as electrically commutated permanent magnet motors.        Use of oversize condenser coils that lower the condenser pressure required.        Use of oversize evaporator coils that raise the evaporator pressure required.        Use of modulating systems that run part of the time at reduced load to increase overall cycle efficiency.        Use of high efficiency blower housings and blower motors to reduce the non-compressor electrical usage.        
However, even with these substantial improvements, obtaining a higher seasonal energy efficiency ratio (SEER) ratings are desired together with less expensive refrigeration systems that do not involve expensive oversize copper and aluminum heat exchangers.
One area where there have been attempts in improving the efficiency in sub-critical point refrigeration cycles is in harnessing the expansion energy that is normally lost across the expansion valve. A theoretical sub-critical point refrigeration cycle that would accomplish this would have a TS diagram such as that shown in FIG. 3.
The refrigeration cycle shown in FIG. 3 is substantially the same as the standard refrigeration cycle discussed earlier and shown in FIG. 1, except that in the refrigeration cycle of FIG. 3, the uncontrolled expansion of the refrigerant that occurs at the expansion valve is instead a controlled expansion with the resultant expansion event being closer to an isentropic event instead of an adiabatic event. The end result of the refrigeration cycle shown in FIG. 3 is that work can be removed from the controlled expansion, and additional refrigeration capacity can be used which is equal to the energy that was removed.
There have been attempts to duplicate the refrigeration cycle shown in FIG. 3 in the past, but for different reasons they were not successful.
U.S. Pat. No. 3,934,424 discloses an attempt at duplicating the refrigeration cycle shown in FIG. 3. However, the requirement of a second compressor that was mechanically coupled to the expansion valve added complexity to the attempt.
U.S. Pat. No. 5,819,554 also discloses an attempt at duplicating the refrigeration cycle of FIG. 3. However, requiring the expansion valve to be directly coupled to the compressor also increased the complexity of this attempt. In addition, putting the cold expansion refrigerant lines out at the compressor could potentially negatively affect the commercialization of the system.
U.S. Pat. No. 6,272,871 also discloses another attempt at duplicating the refrigeration cycle of FIG. 3 through the use of a positive displacement expansion valve. However, this also required a throttle valve being positioned before the expansion device so that the refrigerant moving through the device had a higher vapor content.
U.S. Pat. No. 6,543,238 also discloses an attempt to duplicate the refrigeration cycle of FIG. 3 by using a supercritical point vapor compression refrigerant cycle. This attempt employed a scroll expander, similar to a scroll compressor to expand the supercritical refrigerant. Being a supercritical point cycle, the refrigerant is never incompressible, and therefore easier to manage through the energy recovery system. This system appears practical for very large commercial-type units, but would likely be too complex and too expensive for a residential application.